1. Diameter and Surface Accuracy
The large diameter of the future submillimeter telescopes is key to its cosmological research applications as demonstrated in SCIENCE. The large diameter facilitates a large collecting area (i.e., high sensitivity), less confusion noise, and a small beam size; hence, deeper surveys and reliable identification of multiwavelength counter parts to uncovered subumillimeter sources will be facilitated. If we realize a 50-m-class submillimeter single-dish telescope, we can improve the point source sensitivity by a factor of ~ 20, the confusion noise by a factor of ~ 10, and the beam size by ~ 5 compared with the existing submillimeter dishes in Chile, such as the ASTE 10-m and Atacama Pathfinder Experiment (APEX) 12-m telescopes.
The total surface accuracy (in rms) should be better than 1/16th of the observing wavelength so as to achieve high aperture and beam efficiencies. We take 420 GHz (714 μm) as being the highest observing frequency for full aperture use; hence, the surface accuracy is required to be < 45 μm. To achieve this accuracy under the submillimeter operation conditions, active surface control is essential. Note that the current LST requirements do not include an astrodome.
2. Wide Field of View
In order to conduct extremely large-area (> few 100-1000 sq. deg.) cosmological deep surveys and high-cadence surveys for transient sources, a wide FoV of 0.5 sq. deg. (up to 1 sq. deg.) is required. The FoV will be shared by large-format cameras, imaging spectrometers, and large-format heterodyne receiver arrays. The receiver cabin should have sufficient room for these instruments and related warm optics.
3. Observing Frequency and 1 THz Challenge
ALMA is currently in operation at a frequency range of 80-950 GHz at the Atacama site in Chile. To maximize synergy with ALMA, the LST observations must also be conducted in the same frequency range. However, observations at submillimeter windows, e.g., 690 GHz (450 μm) and 850 GHz (350 μm) are quite tough, because the availability of the excellent atmospheric conditions suited for such short-wavelength observation is limited, even at the Atacama site. If the LST has a significantly large collecting area, it can effectively exploit the "submillimeter weather." The necessity to employ the full aperture of the LST for observations at such high frequencies has quite a significant impact on the telescope design and manufacture, especially as regards the surface and pointing accuracies. Our current plan for the > 420 GHz frequency range is to use the central high-precision area (< 25 μm in rms), which will be ~ 30 m in diameter, and weather that is superior to the submillimeter conditions, i.e., a low PWV (e.g., < 0.5 mm) and less-windy conditions.
4. Pointing Accuracy
The current specification for pointing under the submillimeter observation conditions is < 0.7 arcsec, which corresponds to a gain degradation of ~ 10 % at both 420 (full aperture; FWHM of beam ~ 4 arcsec) and 690 GHz (30 m aperture; also ~ 4 arcsec). Our pointing accuracy goal is ~ 10 % of the FWHM, i.e., ~ 0.4 arcsec, which will be a challenge.
5. Detailed Technical Specifications
Telescope Design Concept
1. Active surface control and adjustable small-size surface panel
A key and basic technology for the LST is an active surface control system. Small adjustable surface panels are required to accurately compensate for gravitational and thermal deformation and to achieve high surface accuracy for each panel. A small panel size is also required from the perspective of the machining capability; a larger panel cannot be machined with high precision. The maximal size is ~ 2-3 m, and several actuators per panel are required for panel adjustment and panel deformation correction.
2. Ritchey-Chretien telescope design
The optical system for the LST requires a large FoV (~ 1°) for survey capability with a large-format (> mega-pixel) camera system using direct detectors. On the other hand, the collecting area and spatial resolution of the 50-m telescope at submillimeter wavelength are also powerful tools for single pointing or smaller (a few arcminutes) FoV instruments (e.g., THz instruments and 1000-pix class heterodyne cameras), which require a Nasmyth cabin for simultaneous operation of several instruments. In this case, a Cassegrain F-number of 6 is appropriate for realizing a compact transmission system with 1° Nasmyth optics.
Aberrations are another limiting factor of the FoV as they decrease the phase coupling efficiency of the optical system. The Ritchey-Chretien (RC) system provides a wider FoV, which is limited by the astigmatism aberration. This is in comparison to the classical design, which is limited by comatic aberration.
3. Preliminary conceptual design
A sample conceptual designs for the LST has been developed and tentative figures of the telescope design have been provided by one of the antenna manufacturers. Some major requirements for the LST (e.g., tentative wide-FoV optics and a large receiver cabin) have been accommodated in this mechanical design, which reveals the LST appearance and the influences of the various specifications on the mechanical design. However, quantitative investigations of the LST performance specifications, such as the surface and pointing accuracies, are not fully included in this design study, and are important items for future investigation. Note that the tentative surface error budget for the LST is also shown in Table 3.